1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine including a color digital copier, a printer including a multifunction printer (MFP) and a laser printer, a facsimile machine, a plotter, or a multifunction product having these functions, and, more particularly to a multi-color image forming apparatus that forms a color image by superposing toner images of a plurality of colors. The present invention also relates to an optical scanning apparatus used as a writing system in the image forming apparatus, and a scanning-line-change correcting method.
2. Description of the Related Art
In recent years, to correspond to high speed and high image quality of color image forming apparatuses, a digital copier and a laser printer have been in practical use, which simultaneously expose a plurality of optical beams on four photoconductors arranged in a transport direction of output paper, and images developed by a developing device that stores and supplies developers of different colors, that is, yellow, magenta, cyan, and black are sequentially transferred and superposed to form a color image.
Generally, such an image forming apparatus has a plurality of scanning units corresponding to each of the optical beams (“beams”). However, a large space is required for arranging the scanning units, thereby increasing the size of the image forming apparatus. Accordingly, there has been proposed a method in which beams are made to enter a single deflector and scanned, and imaging lenses are arranged in a stacking manner (for example, see Japanese Patent Application Laid-Open No. H4-127115).
Further, to compensate the problem in Japanese Patent Application Laid-Open No. H4-127115, there has been proposed a method in which an imaging unit that makes a plurality of beams enter a single deflector to scan the beams and forms an image on a corresponding photoconductor is provided for each beam, and an optical elements constituting the imaging units are integrally formed by being stacked in layers in a sub-scanning direction (for example, see Japanese Patent Application Laid-Open No. H10-148777). According to the technique disclosed in Japanese Patent Application Laid-Open No. H10-148777, an interval for stacking the optical deflectors can be reduced, or one polygon mirror can function as the optical deflector. Accordingly, load of the motor for rotating the polygon mirror can be reduced, and the apparatus can be made compact.
One major problem in the color image forming apparatus that incorporates the optical scanning apparatus therein is out-of-color registration. That is, in the color image forming apparatus, a plurality of optical beams are simultaneously exposed on four photoconductors arranged in a transport direction of output paper, and images developed by the developing device for different colors (yellow, magenta, cyan, and black) are sequentially transferred and superposed to form a color image. Accordingly, scanning misregistration of the beams on a surface to be scanned of each photoconductor directly appears as out-of-color registration of respective colors.
It has been found that the scanning misregistration, which causes out-of-color registration, is caused mainly by a generatrix curvature of the imaging lens constituting the imaging unit or an imaging optical system (f-theta lens optical system or the like), which images the beams deflected by the optical deflector on the corresponding photoconductor respectively.
Particularly, when the imaging lens is a lens made of plastic (“plastic lens”), outline deformation easily occurs due to its machining characteristic. If the outline deformation occurs in the sub-scanning direction, a generatrix curvature occurs in the same direction, that is, scanning misregistration in the sub-scanning direction occurs on the photoconductor surface.
To avoid the disadvantage, it is necessary to improve the outline accuracy by considerably extending a forming cycle of the plastic lens, or providing a plastic lens position-adjusting mechanism to adjust the outline deformation. This causes a cost increase and complicates the entire apparatus, which is not desirable.
These disadvantages were not considered as problems before, however, they have recently surfaced as problems due to demands for high speed and improvements of image quality in image forming apparatuses.